When distances are measured by means of electromagnetic waves, it is of the greatest importance, if high accuracy is to be achieved, that the measuring instrument is corrected for the electromagnetic wave velocity in air, since this velocity varies with the air pressure and temperature. The velocity also varies with the humidity of the air, although the effect of this is relatively small. To obtain this correction factor until now the temperature of the air has been measured by means of a thermometer and its pressure by means of a barometer. Subsequently the correction factor has been obtained by calculation work from these data. In certain cases also a so-called nomogram has been available, where it has been possible by entering the known values of temperature and pressure into the scales directly to read off the correction factor.
The present invention relates to a method and a device which allows the direct reading of the correction factor on a scale, without first using a thermometer or barometer and subsequently carrying out the calculation work mentioned above or go into a tabular compilation. In accordance with a further development of the invention it is not even necessary to read a scale, but the device in accordance with the invention can directly correct the measuring instrument, in which the correction factor is to be applied.
The measuring instrument which is used for measuring the distance and which used modulated light is so accurate that it requires an estimation of the temperature which is better than 1.degree. C and of the pressure which is better than 3 mm Hg. These variables each give a correction of approx. 10.sup..sup.-6 at 760 mm Hg and 20.degree. C. The humidity makes a contribution to its error which is usually less than 10.sup..sup.-6, which can thus normally be neglected.
It is a known fact that the abovementioned correction factor is proportional to the air pressure and inversely proportional to the absolute temperature of the air. From the general gas law it is known that the inverted value of a volume of a quantity of air is proportional to its pressure and inversely proportional to its absolute temperature. The method and device in accordance with the present invention make use of these relations.